Much of my recent work has been focused on advancing geoarchaeology in New York State, but I have been able to work some on projects elsewhere. In the mid-1990s I noticed that much of the color variation once described as “basket-loading”, in portions of ca. 2,000-year-old Illinois Hopewell mounds, were actually color variations inherent to their nature of being sod blocks, turned upside down when emplaced as mound fill. This primary geoarchaeological observation has helped to raise many interesting research questions about the role of mound building within Illinois Hopewell societies, and how and why these and similarly constructed mounds elsewhere were built in this way. Further archival research suggests that the use of sod blocks at this time minimally extended to some mounds in Michigan and North Dakota.

View to the west of Monks Mound from
Mound 34 at Cahokia.

Later period mounds were constructed in fundamentally different ways, and I have been very fortunate to be a part of the team of scientists assembled by John Kelly and Jim Brown to investigate the Mississippian Mound 34, at Cahokia Mounds State Historical Site in Illinois (www.cahokiamounds.com). Much of the geoarchaeological work for this project has been directed at simply trying to figure out what the original landscape and soils may have looked like in this part of the very large Cahokia site, by gathering baseline geological data by coring and describing available profile exposures. Extensive earth-moving at Cahokia has been previously documented by many, and in the Mound 34 area there are some remaining geological sections that give us an idea of the extent of earth-moving that took place prior to the construction of Mound 34. By paying close attention to the soils and soil materials within the many elements of this mound, the geoarchaeological study is beginning to ferret out some interesting archaeological attributes. One result has been to show that a distinctive, thin but laterally extensive (stratiform) fill, that outwardly looks like an in situ soil A horizon, is more likely to owe all its characteristics to the source from which it was quarried as mound fill. Recognizing that this deposit, containing feasting debris, was actually removed from some other place, to be emplaced here, lends support to the notion that the source of many mound fills likely carried symbolic meaning to the builders of the mound. Another recent finding is the presence of sealed mound fill units displaying well-developed soft-sediment deformation features. Whether or not the introduction of water to mound fill deposits in this context is deliberate or simply incidental raises a number of additional fascinating research questions to explore.

Dr. Leighton’s field notebooks are archived
in the library of the Illinois State
Geological Survey.

To place the geoarchaeological work at Mound 34 into historical perspective, I am closely re-examining the archived field notes and published work of the late Morris M. Leighton, former Chief of the Illinois State Geological Survey, and a true pioneer in the interpretive use of soils in both Quaternary stratigraphic studies and archaeology. Early in his career (in the 1920s) Leighton was called upon to settle the now-hard-to-comprehend dispute of whether or not the mounds at Cahokia were humanly-made, or whether, as some prominent geologists held at the time, they were natural erosional outliers of the Mississippi River bluffs. A couple of the geological arguments put forth then included the observations that the height of the largest mound, Monks Mound, approximated the heights of the distant bluff faces (an interesting fact to reflect upon in its own right, irrespective of its earlier misinterpretation), and that mound fills were often observed to be “stratified.” To counter the stratified argument, Leighton proposed the use of the term “stratiform”, to disassociate the word stratified from perceived genetic connotations of sediment laid down by water or wind. Leighton went on to show that stratiform fills must have been a deliberate effect created by the builders. The many difficulties and challenges of demonstrating whether some mound fill attributes have a geogenic or an anthropogenic origin remain with us today.